Apparatus of the type set forth above is disclosed in U.S. Pat. No. 4,099,507. In this German publication, as in the present invention, a sequence of auxiliary pulses is furnished by a sensor sensing, for example, markings on a rotating shaft of an internal combustion engine. As in the present invention, a counter is provided for counting these auxiliary pulses and a decoding stage furnishes a signal when the count on the counter has reached a predetermined count. However, in the system disclosed in DT-OS No. 2,523,388, the duty factor (ratio of pulse width to period of the control pulse sequence) is not changed as a function of current build-up time through the impedance element but is determined by the output of a timing circuit. For internal combustion engines in particular this arrangement has the disadvantage that changes in supply voltage cause changes in the energy stored in the ignition coil and therefore result in ignition pulses having variable energies. A further disadvantage is that the pulse width of the control pulses of the known apparatus must be so designed that sufficient ignition energy will be available even when the supply voltage is particularly low. When the supply voltage is higher, the ignition current will then be greater than required, leading to excessive current flow through the primary winding of the ignition coil. This in turn causes high power losses and possibly destruction of the circuit elements in the primary circuit. In addition, the circuit elements in the primary circuit of the ignition coil must be manufactured to very small tolerances so that the current build-up time specified by the timing circuit results in sufficient energy stored in the ignition coil. It is also very difficult to compensate the temperature variations in the known apparatus.